Vitamin B12 is a dietary essential, a deficiency of which results in defective synthesis of DNA in any cell in which chromosomal replication and division are taking place. Since tissues with the greatest rate of cell turnover show the most dramatic changes, the hematopoietic system is especially sensitive to vitamin B12 deficiencies. An early sign of B12 deficiency is a megaloblastic anemia. Dietary B12, in the presence of gastric acid and pancreatic proteases, is released from food and salivary binding protein and bound to gastric intrinsic factor. When the vitamin B12-intrinsic factor complex reaches the ileum, it interacts with a receptor on the mucosal cell surface and is actively transported into circulation. Adequate intrinsic factor, bile and sodium bicarbonate (suitable pH) all are required for ileal transport of vitamin B12. Vitamin B12 deficiency in adults is rarely the result of a deficient diet; rather, it usually reflects a defect in one or another aspect of this complex sequence of absorption. Achlorhydria and decreased secretion of intrinsic factor by parietal cells secondary to gastric atrophy or gastric surgery is a common cause of vitamin B12 deficiency in adults. Antibodies to parietal cells or intrinsic factor complex also can play a prominent role in producing deficiency. A number of intestinal diseases can interfere with absorption. Vitamin B12 malabsorption is seen with pancreatic disorders (loss of pancreatic protease secretion), bacterial overgrowth, intestinal parasites, sprue, and localized damage to ileal mucosal cells by disease or as a result of surgery. The recommended daily intake of vitamin B12 in adults is 2.4 μg.
There are four main forms of vitamin B12: cyanocobalamin: hydroxocobalamin, methylcobalamin and adenosylcobalamin. Methylcobalamin and adenosylcobalamin are unstable and damaged by light. They are therefore unsuitable for use in dietary supplements or pharmaceuticals and are not essential since they can be formed from cyanocobalamin or hydroxocobalamin within the body. The main form of vitamin B12 found in food is hydroxocobalamin. The main form used therapeutically and in nutritional supplements is cyanocobalamin, chosen because it is the most stable form and therefore easiest to synthesize and formulate.
Because deficiencies of vitamin B12 are generally caused by the inability of the vitamin to be absorbed in the small intestine due to a breakdown in the vitamin B12-intrinsic factor complex transport mechanism, vitamin B12 must therefore be administered systemically. Currently, therapeutic amounts of cyanocobalamin are administered by intramuscular or deep subcutaneous injection of cyanocobalamin. However, patients must return to the physician's office periodically to receive additional injections to maintain their levels of vitamin B12. However, an intranasal gel cyanocobalamin preparation, NASCOBAL® is currently being marketed in which cyanocobalamin is administered intranasally as maintenance vitamin B12 therapy. However, many patients find the consistency of the intranasal gel unpleasant and would prefer to have administered intranasally a low viscosity spray containing cyanocobalamin.
The prior art suggests that for vitamin B12 to be absorbed intranasally in therapeutically beneficial amounts, the concentration of the B12 in solution must either be greater that 1% by weight, see Merkus; U.S. Pat. No. 5,801,161 or be administered intranasally in a viscous gel, Wenig, U.S. Pat. No. 4,724,231 so that the gel remains in the nostril for an extended period of time. In fact Wenig states that B12 administered intranasally in a low viscosity solution is not in contact with the nasal mucosa long enough for a sufficient period of time to permit useful absorption. Wenig claims that most of the B12 is wasted if the solution has a low viscosity. Merkus developed intranasal formulations of hydroxocobalamin having a concentration of hydroxocobalamin greater than 1%, however hydroxocobalamin is not very stable and thus has a short shelve-life. Merkus chose hydroxocobalamin because cyanocobalamin is not soluble in an aqueous solution at concentrations greater than 1%.
U.S. Pat. No. 4,525,341, Deihl, discloses a method of administering vitamins intranasally but do not enable a specific formulation containing only cyanocobalamin. International Patent Application No. PCT/US86/00665, publication no. WO 86/05987, discloses nasal spray composition containing vitamin B12 as cyanocobalamin. However, the specific spray formulations all contained a mercury compound as a preservative, however the disclosure did require the presence of mercury compounds. Other preservatives were also mentioned including benzalkonium chloride and chlorobutanol. As was stated above, an intranasal gel containing cyanocobalamin, NASCOBAL®, is currently being produced and marketed by Nastech Pharmaceutical Company Inc. of Bothell, Wash. It is very effective in maintaining levels of vitamin B12 for patients who have been deficient in the past but have recovered their levels of B12 through intramuscular injections. However, a number of patients find the consistency of the gel unpleasant in their nose, and would prefer an intranasal formulation that has a lower viscosity and is free of mercury compounds. Thus, there is a need to produce a pharmaceutically stable aqueous solution of cyanocobalamin that has a low viscosity, is free of mercury compounds and has sufficient bioavailability to be used as a maintenance therapy for vitamin B12.